One of the perks of winning an Oscar for screenwriting is that you might get a chance to try your hand at directing — and one of the perils of winning an Oscar for screenwriting is that you might get a chance to try your hand at directing.

Such is the situation of William Monahan, who took home an Academy Award for penning “The Departed” five years ago and parlayed his success into a deal to move into the director’s chair. “London Boulevard,” his debut effort, has more than a hint of his earlier triumph. Monahan, however, is not yet Martin Scorsese.

The bumpy “Boulevard” follows recently paroled Mitchel (Colin Farrell) through the mean streets and posh playgrounds of the English capital city. Although Mitchel claims to be taking the straight and narrow path, he still ends up rubbing shoulders with unsavory old pals, like the hopped-up Billy (Ben Chaplin), who keeps trying to lure him back to the shady side with promises of quick money.

The ex-con-at-a-crossroads concept wasn’t exactly springtime fresh even when James Cagney and Edward G. Robinson were at their peak, but Monahan adds a twist. Mitchel lucks into a potentially lucrative gig handling security for Charlotte (Keira Knightley), a world-famous actress who’s recovering from a shattered marriage and a nervous breakdown.

While her traffic-stopping, come-hither portrait is plastered on billboards all over town, Charlotte spends her days in her shadow-filled Holland Park mansion, hiding from paparazzi prowlers; they’re almost as abrasive and nasty as Mitchel’s old cohorts.

Although Monahan makes excellent use of a soundtrack crammed with killer tracks from The Yardbirds and The Rolling Stones and director of photography Chris Menges’ elegant camera work makes the grimiest corners of London seem exotic and inviting, “London Boulevard” turns out to be a long way from “Sunset Boulevard.”

Farrell and Knightley do everything they can to give substance to their wafer-thin characters — he’s all simmering anxiousness, while she focuses on Charlotte’s emotional fragility — but the relationship between them is never quite convincing, and the pokey screenplay (based on a novel by Ken Bruen) is sorely lacking in insights into either the underworld or the glossy, high-pressure environment in which Charlotte lives.

With no genuine driving force behind the story, the actors attempt to compensate with sheer personality. For instance, the masterful British character actor Ray Winstone growls and grimaces as Mitchel’s nemesis, a crime boss known as Gant, and generates a few sparks with Farrell, although many of their interactions seem like carbon copies of the Jack Nicholson/Leonardo DiCaprio duels in “The Departed.”

Meanwhile, Monahan the director often gives Monahan the screenwriter a bit too much rope.

Lines like “if it wasn’t for Monica Bellucci, she’d be the most raped actress in European cinema” may have looked saucy/sassy on the page, but they land with a thud on the screen, even when the redoubtable David Thewlis (as Charlotte’s daffy assistant/housekeeper/babysitter) is delivering them.

Similarly, Anna Friel puts a lot of spiky allure and submerged sorrow into Mitchel’s unbalanced, often sloshed sister, Briony, only to be stranded in a subplot that ultimately goes nowhere.

There’s a surplus of talent on-hand in “London Boulevard,” but Monahan doesn’t package it particularly well.

The movie remains a slick collection of potentially potent pieces that won’t coalesce into a full-bodied drama — and that’s the way this tough cookie crumbles.